The present invention relates to improvement in an automatic screw or nut tightening tool which is called a nut runner or nut setter and employed, for example, for tightening screws of an engine cover of a car at a time. In particular, the invention pertains to improvement in a control circuit of a motor for driving such a nut runner.
An air nut runner operated by compressed air has heretofore been used, but it cannot produce a screw tightening force of high accuracy. As a nut runner for tightening screws or nuts at a high accuracy, a motor-driven nut runner using a DC motor has been proposed and is now placed on the market; however, the DC motor has the fatal defect of wear of its brushes. To avoid this, a DC brushless nut runner has been proposed which is free from the problem of wear of the brush; however, the nut runner of this type is disadvantageous in that the motor structure and the control circuit therefore are complex, and in that since a permanent magnet is used as the rotor, the volume of the motor is large relative to the motor output power, making it difficult to arrange a lot of such nut runners at a small pitch.